As this type of device, for example, a waste-heat reusing device disclosed in Patent Document 1 has been known. The waste-heat reusing device disclosed in Patent Document 1 has: a Rankine cycle which is equipped with a pump, a heater, an expander, and a condenser; a bypass flow passage which bypasses the expander; and a bypass valve that opens and closes the bypass flow passage. When starting up the Rankine cycle, refrigerant is first circulated with the bypass valve open, and when a temperature of gaseous-phase refrigerant on the inlet side of the expander becomes a predetermined temperature or higher, the bypass valve is closed, and operating rotational speeds of the expander and the pump are made to increase. Moreover, after closing the bypass valve, when the pressure difference (expander pressure difference) between the inlet and the outlet of the expander becomes a predetermined pressure difference, the start-up completion of the Rankine cycle is determined.